1. Field of the Invention
This invention pertains generally to circuit interrupters and, more particularly, to such circuit interrupters employing one or more poles each of which includes a vacuum interrupter. The invention also pertains to apparatus for monitoring a vacuum interrupter for leakage or loss of vacuum of the vacuum envelope.
2. Background Information
Circuit interrupters provide protection for electrical systems from electrical fault conditions such as, for example, current overloads, short circuits and abnormal level voltage conditions. Typically, circuit interrupters include a spring powered operating mechanism which opens electrical contacts to interrupt the current through the conductors of an electrical system in response to abnormal conditions, although a wide range of driving mechanisms may be employed.
Circuit interrupters, such as, for example, power circuit breakers for systems operating above about 1,000 volts typically utilize vacuum interrupters (VIs) as the switching devices. Vacuum circuit interrupters include separable main contacts disposed within an insulating housing. Generally, one of the contacts is fixed relative to both the housing and to an external electrical conductor which is interconnected with the power circuit associated with the circuit interrupter. The other contact is moveable. In the case of a vacuum circuit interrupter, the moveable contact assembly usually comprises a stem of circular cross-section having the contact at one end enclosed within a vacuum chamber and a driving mechanism at the other end which is external to the vacuum chamber. An operating rod assembly comprising a push rod, which is fastened to the end of the stem opposite the moveable contact, and a driving mechanism provide the motive force to move the moveable contact into or out of engagement with the fixed contact.
VIs are typically used, for instance, to reliably interrupt medium voltage alternating current (AC) currents and, also, high voltage AC currents of several thousands of amperes or more. Typically, one VI is provided for each phase of a multi-phase circuit and the VIs for the several phases are actuated simultaneously by a common operating mechanism, or separately by separate operating mechanisms (and separate auxiliary switches).
A leaking VI is extremely rare. However, if a leak does occur, then the VI may not be able to interrupt a corresponding current flow and/or it may not be able to withstand the required voltage when its separable contacts are open. Loss of vacuum due to leakage of a VI is one of the major failure modes of vacuum circuit interrupters. Currently, there is no known cost effective method or technology to monitor the vacuum status inside the VI.
A known technology to measure or detect the vacuum integrity of a VI is to use two potential transformers to measure the voltage change across the VI. As the pressure inside the VI gets higher, the breakdown voltage decreases based on the “Paschen Curve”. The change of the breakdown voltage while the contacts are open and a sufficiently high voltage applied across the contacts provides an indication of the vacuum integrity of the VI. However, this can hardly be implemented into a circuit breaker or a circuit breaker enclosure due to the relatively large size of the potential transformers.
Paschen's Law essentially states that the breakdown characteristics of a gap are a function (generally not linear) of the product of the gas pressure and the gap length, usually written as V=f(pd), wherein p is the pressure and d is the gap distance. FIG. 1 shows the “Paschen Curve” for air, in which two flat parallel copper electrodes are separated by 0.1 inch, for pressures between about 3×10−2 torr and 760 torr. As the pressure is reduced below a few torr, the curve of breakdown voltage versus pressure reaches a minimum, and then, as pressure is further reduced, rises steeply again.
There is a need, therefore, to detect a loss of vacuum or a leaking vacuum of a VI in a cost effective manner.